The present invention relates to a power supply system supplying a load with single or multiphase AC electrical power, from a mains power supply, the system comprises an auxiliary back-up power supply capable of replacing the main power supply, in the event of failure of the latter, a static contactor located between the load and the mains power supply, a detection device for detecting almost instantaneously when the mains power supply goes outside preset tolerances and for controlling the opening of the static contactor, and means for detecting the return of the mains power supply to within said tolerances.
It is state of the art to use a bank of batteries operating an inverter as back-up power supply. A static contactor, which is placed between the normal mains power supply and the load and which is closed in normal operation, opens automatically in the event of failure of this mains power supply in order to disconnect it from the load immediately, and it is then the inverter which supplies electrical power directly to this load. When this failure, which can normally only be temporarily on account of the limited capacity of the batteries, has been cleared, this static contactor recloses and the mains power supply again supplies the load, whereas it generally recharges the batteries via the inverter which operates in a reversible manner.
When the use is sensitive, it is important for the load to be isolated from the mains power supply very quickly in the event of failure of the latter. Consequently, a failure of the mains power supply has to be detected very quickly and a static contactor with high-speed opening has to be used.
Although the second condition does not cause any problem at present, due to the use for this static contactor of semiconducting components with turn-off capability (transistors, GTO . . . ) or turn-off assisted by an auxiliary circuit (thyristors), the same is not the case for high-speed detection of the mains power supply going outside its tolerances, which is the basis of the static conductor turn-off order. As the mains in fact supplies an AC voltage, it is not sufficient, for almost instantaneous detection of a failure, either to measure its peak voltage or to take into consideration the mean value of its rectified voltage, these two conventional processes generally giving rise to a detrimental delay.